1. Field of the Invention
The present invention relates to a mass spectrometer which has a glow discharge ionizer and a photoionizer that are coupled to a mass detector(s) by quadrupole ion traps.
2. Background Information
Terrorists have been known to use explosives to hijack commercial aircraft. For this reason, there has been a desire to provide an explosive detection system that can be operated "on-site" at an airport terminal. An on-site detection system must be capable of detecting extremely low concentrations of an explosive(s) material in a relatively fast time frame to minimize the time delays in air travel for the passengers.
U.S. Pat. No. 5,854,431 issued to Linker et al. and assigned to Sandia Corporation ("Sandia") discloses a preconcentrator system that generates a flow of air to dislodge explosive material from a passenger. The dislodged explosive material is captured by a screen of the system. The air flow across the passenger is temporarily terminated to allow the captured explosive material to be removed from the screen by a secondary flow of air. The explosive material removed from the screen is directed into a particle detector. The preconcentrator disclosed in the Sandia patent increases the concentration of explosive material provided to the detector.
U.S. Pat. No. 4,849,628 issued to McLuckey et al. ("McLuckey") discloses a mass detection system that can detect relatively low concentrations of a trace molecule(s). McLuckey utilizes a glow discharge ionizer which ionizes an "atmospheric" sample. Providing an air sample at atmospheric pressures increases the density of the sample and the number of ionized molecules. Increasing the number of ions improves the sensitivity of the detector.
The glow discharge ionizer includes a pair of electrodes separated by a chamber. A voltage potential is created between the electrodes to induce a glow discharge which ionizes a gas sample within the chamber. The glow discharge ionizer of McLuckey is coupled to a quadrupole mass spectrometer that can detect a trace molecule such as an explosive material.
The quadrupole mass spectrometer includes a scanning circuit which provides a continuously varying voltage field across the poles of the spectrometer. The continuously varying voltage field sequentially ejects ionized molecules from the quadrupole to a detector. The excitation circuit and detector can be coupled to a computer which correlates detected molecules with the excitation voltage. Explosive materials will provide detection at a predetermined voltage(s). The computer can correlate detection with an explosive material and inform an operator that an explosive has been detected.
Quadrupole mass spectrometers are relatively slow because of the time required to vary the excitation voltage to sequentially eject the ionized trace molecules. The prior art does include time of flight mass spectrometers, which simultaneously accelerate all of the ionized molecules toward a detector and then detect the different times when the molecules arrive. The mass of the molecules varies with the different arrival times. Time of flight mass spectrometers are not effective when used with a continuous ionization source such as a glow discharge ionizer. It would be desirable to provide a monitor that can quickly detect trace molecules in relatively low concentrations.
Glow discharge ionizers are efficient in ionizing molecules with high electron affinity but are not generally effective for molecules with low ionization potentials, which generally have low electron affinity. It would also be desirable to provide a monitor that can quickly detect a variety of different trace molecules in relatively low concentrations. For example, it would be desirable to provide an on-site airport terminal detector that can detect explosives as well as other threats and contraband such as chemical weapons and drugs.